Implantable material and method of preparing same

ABSTRACT

A composition of material suitable for in vivo implantation to provide a wear surface which composition includes carbon fibers, perfluorocarbon fibers, fluorinated carbon fibers, fluorinated carbon particles, fluorinated hydrocarbon fibers, fluorinated hydrocarbon particles, polytetrafluoroethylene fibers or combinations thereof and polytetrafluoroethylene resin alone or with a high molecular weight polyethylene all of which composition is processed to align a substantial portion of the fibers with the wear surface. The preferred method of preparing such composition of material includes the steps of mixing, filtering, compressing, rolling, sintering and drying.

CROSS REFERENCE TO RELATED APPLICATIONS

This is a division of application Ser. No. 758,499, filed Jan. 11, 1977,now U.S. Pat. No. 4,178,532, which is a continuation-in-part of my priorcopending application Ser. No. 300,246, filed Oct. 24, 1972 nowabandoned.

SUMMARY

The present invention relates to an improved composition of mattersuitable for in vivo implantation as a wear surface and to the method ofpreparing such composition of material.

An object of the present invention is to provide an improved compositionof material suitable for in vivo implantation which exhibits highresistance to wear.

Another object is to provide an improved wear material which has lowfriction and is highly resistant to wear.

A further object is to provide an improved wear material which whensubjected to frictional movement has minimum galling and releasing ofparticles responsive thereto.

Still another object is to provide an improved method of preparing awear material of fibers and resin so that the fibers are generallyaligned with the wear surface.

A still further object is to provide an improved composition of materialfor in vivo implantation which may be sterilized by the usualsterilization procedures and apparatus, such as a steam autoclave,without being adversely affected thereby.

Still another object is to provide an improved composition of materialfor in vivo implantation which in addition to the above desired featuresof having low friction and substantial resistance to wear is white incolor.

These and other objects and advantages of the present invention arehereinafter set forth and explained.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

A wear material suitable for in vivo implantation is adapted to besterilized in a steam autoclave, does not include anything which wouldbe toxic or cause a body reaction and has a surface exhibiting lowfriction and high resistance to wear.

One preferred form of wear material of the present invention includescarbon fibers and a resin such as polytetrafluoroethylene which areprepared in a manner to position the carbon fibers in generally parallelrelationship to the surface of the wear material.

This wear material which has been found suitable for implantation ashereinbefore described has a composition of more than 15 percent byvolume to 65 percent by volume carbon fibers and particles and less than85 percent by volume to 35 percent by volume of polytetrafluoroethyleneresin such as is marketed by Du Pont as their TFE resin.

A preferred composition of material for implantation is 40 percent byvolume fibrous carbon or graphite and 60 percent by volume of the TFEfluorocarbon polymer. A composition which has exhibited excellent wearproperties and low friction is one containing 30 percent by volumefibrous carbon or graphite, 10 percent by volume particulate carbon orgraphite and 60 percent by volume TFE polymer. Generally it is preferredthat the ratio of total fibrous and particulate carbon to fibrous carbonbe from 1 to 1 to 5 to 1.

A specific preferred composition providing 40% by volume carbon fiberfiller includes graphite fiber sold by Carborundum Company under thename GY 2F, 12.0 grams; Du Pont Teflon, TFE-6 Resin, 2.76 grams; and DuPont Teflon, TFE-7 Resin, 24.8 grams. Another useful compositionproviding 40% by volume fluorinated carbon fiber includes fluorinatedgraphite fiber prepared by Marchem Company, Houston, Texas, 19.6 grams,Du Pont Teflon TFE-6 Resin, 2.76 grams, and Du Pont Teflon, TFE-7 Resin,24.8 grams. Another useful composition includes graphite fiber sold byCarborundum Company under the name GY 2F, 10.8 grams, particulate carbonsold by Biocarbon, Tarzana, California (technically described asvitreous carbon frit), 3.6 grams, Du Pont Teflon, TFE-6 Resin, 2.8grams, Du Pont Teflon, TFE-7 Resin, 25 grams. This Composition provides30% by volume carbon fiber filler and 10% by volume particulate carbonfiller.

The wear composition is prepared by mixing the resin and carbon orgraphite with a suitable diluent such as isoparaffinic hydrocarbon in ahigh speed, high shear mixer. The amount of diluent is adjusted to thesize of the mixer. For example, in a mixer of 500 milliliters, 375milliliters of diluent is used for dry ingredients weighingapproximately 50 grams. Mixing is carried out until a complete uniformslurry is produced.

The mixed slurry is filtered. The filtration is preferred to be byvacuum filter such a Buechner funnel, and should proceed until theresidual diluent left in the filter cake is less than approximately 20percent by weight.

Following filtration, the filter cake is placed between the platens of aheated press and is compressed at levels of from 500 to 3,000 psi and ata temperature between 100° F. and 250° F. for periods from one to fiveminutes. The conditions are adjusted so that the diluent level aftercompression is less than fifteen percent by weight. Optionally, thecompressed filter cake may be dried in an oven to remove all diluent attemperatures between 150° F. and 500° F. and for times up to severalhours.

Next, the compressed filter cake is run through the nip of heated rollswhich are heated to a temperature between 100° F. to 250° F. Thistemperature is adjusted to the particular volatility of the diluent. Thethickness of the cake is reduced in decrements of approximately20/1000ths of an inch to a thickness between 20/1000ths and 60/1000thsof an inch.

When the desired thickness is reached, the temperature of the rolls iselevated to between 280° F. and 360° F. and the thickness of thematerial during each subsequent pass is reduced to 1/2 of its thickness.To maintain the desired thickness, the sheet of material is doubledafter each pass and then is run through the next pass at 90° to theprevious pass. This procedure may be carried out from four to eighttimes depending on the apparent toughness of the product at a givenstage of rolling. It may be desirable to roll down to 1/2 originalthickness in steps of 5/1000ths or 10/1000ths of an inch.

It should be noted that the sheet material produced by the precedingrolling step can be formed into particular shapes if desired bycompression molding, vacuum drawing, and other shaping operations atrelatively low pressures and the shaped material can then be compressionsintered in an appropriate mold to retain such shape.

When the rolling is completed, the material is sintered at a temperaturefrom 610° F. to 680° F. for periods from thirty minutes to several hoursdepending on the thickness of the stock. It should be noted that if theproduct contains residual diluent which is slow to evaporate, extendedperiods of drying at temperatures from 300° F. to 500° F. may be usedprior to sintering to assure removal of the diluent. When fluorinatedparticulated filler are used, it is recommended that the solvent beremoved before rolling since difficulty may be encountered withblistering of the sheet stock during sintering if the diluent is notremoved. In certain materials which are difficult to dry it may bedesirable to vacuum dry the filter cake before rolling to avoid theblistering problem.

If desired, the sintering step may be a pressure sintering step in whichthe material is placed between the platens of a heated press at atemperature in the range from 640° F. to 740° F., preferably 700° F. ata pressure in the range from 50 p.s.i. to 5,000 p.s.i., preferably 2,000p.s.i. and for a period of time from 1 to 30 minutes, preferably fiveminutes. The material is moved directly from the press and rapdilycooled by forced air connection or immersion in a room temperature waterbath. This pressure sintering may be used in place of or in addition tothe above described compression sintering step. This technique may alsobe used to prepare multi-ply laminates from unsintered stock.

The preferred forms of such wear material include the fibrous carbon.This material has improved wear properties and low friction. It isbelieved that the reason for such improved properties results from theorientation of the carbon fibers to a position generally parallel to thewear surface. In such position, the carbon fibers would not have atendency to break off and thereby create an extreme wear problem andfurther since the carbon fibers have a low coefficient of friction, theexposure of the carbon fibers on the wear surface would not cause adrastic increase in friction as might be expected with other materials.

The improved wear material may use for its matrix any perfluorinatedhigh polymer such as polytetrafluoroethylene (Teflon TFE), apolyhexafluoropropylene or a co-polymer of hexafluoropropylene andtetrafluoroethylene which is commercially available from Du Pont underthe name Teflon FEP resin or mixtures of such polymers.

Also if desired, the reinforcing additive may be carbon fibers,perfluorocarbon fibers, fluorinated carbon fibers, fluorinated carbonparticles or a combination of two or more of the foregoing types ofcarbon fibers. The use of the fluorinated carbon fibers is advantageoussince the fluorination reduces the critical surface tension of thefibers to the order of 20 dynes per centimeter thereby more closelymatching the critical surface tension of the matrix polymer. Thismatching provides substantially greater adhesion between the matrixpolymer and the reinforcing additive. The fluorinated carbon fibers arealso believed to be advantageous as the fluorination changes the surfaceof the carbon fibers to further reduce the friction in the wearmaterial.

In such matching of surface tension of the reinforcing additive orfiller and the matrix polymer they may both be selected to have the samecritical surface tension or either may be processed to match the other.For example, polytetrafluorocarbon fibers and particles may be used witha perfluorocarbon matrix. Examples of such processing of the latter typeof material would be fibers and particles of carbon, hydrocarbon orother organic material (such as polyimide) which have been treated tohave a surface fluorination. The process of fluorinating the surface ofcarbon materials is known and is disclosed in the J. L. Margrave et alU.S. Pat. No. 3,674,432, issued July 4, 1972 and in the article "MethodHarnesses Direct Fluorination" appearing in the Jan. 12, 1970 edition ofthe Chemical and Engineering News.

It has been found that a suitable material may be provided by acomposition of a filler element such as a fluorinated carbon or afluorinated hydrocarbon or a fluorinated organic material (such asfluorinated polyimide) in either particulate or fibrous form and asintered perfluorinated high polymer resin retaining said filler elementin the structure.

Another preferred composition of wear material may be produced utilizinga high molecular weight polyethylene (HMWPE) in certain formulations.For example, very suitable materials were produced from the followingrecipes:

(a)

20% (Vol.) carbon fiber

20% (Vol.) Teflon TFE fiber

20% (Vol.) Teflon TFE-6 resin, and

40% (Vol.) HMWPE

(b)

20% (Vol.) carbon fiber,

15% (Vol.) Teflon TFE fiber,

15% (Vol.) Teflon TFE-6 resin, and

50% (Vol.) HMWPE

These formulations are bonded at a temperature in the range of 100° F.to 150° F. (preferably 120° F.) are rolled at a temperature in the rangeof 230° F. to 300° F. (preferably 270° F.) and are sintered at atemperature in the range of 500° F. to 540° F., a pressure in the rangeof 500 to 2500 psi and for a period from one to four minutes.

Still another formulation which provides a white, tough material havinglow friction and low wear is a combination of 50% to 85% (Vol.) HMWPEand equal parts of Teflon TFE fiber and Teflon TFE-6 resin. Thiscomposition is rolled at a temperature in the range from 200° F. to 300°F. and is sintered at a temperature in the range from 350° F. to 400°F., at a pressure between 200 and 1,000 psi and for periods up to twominutes.

It has been unexpectedly found that with such high percentages of theHMWPE the absence of the Teflon TFE resin appears to prevent adequatecohesion of the structure during rolling. It is postulated that theresin provides internal lubrication which allows the proper cohesion ofthe composition during the rolling process. The addition of at least 0.5percent (Vol.) of this resin appears to be adequate for the desiredcohesion.

As used herein "high molecular weight polyethylene" shall meanpolyethylene having a molecular weight greater than about one millionsuch as the Hercules Corporation product sold under the trademark "HiFax 1900".

The method of producing these additional compositions of wear materialof the present invention are substantially as hereinabove describedexcept that formulation and temperature ranges are modified aspreviously mentioned with respect to each of said formulations to adjustfor the particular properties of each component and to produce thepreferred composition of material with each formulation.

The wear material of the present invention has a particular applicationfor in vivo implantation when bio-compatible materials are used but mayhave other applications not limited to biocompatible materials.

From the foregoing it can be seen that the improved wear materialresults from a combination of a filler element such as carbon fibers orpolytetrafluoroethylene fibers in a matrix of a perfluorinated highpolymer resin alone or in combination with a high molecular weightpolyethylene resin with the orientation of the fibers being controlledby the method of preparing the material so that they are generallyparallel to the material surface which orientation decreases frictionand increases wear resistance of the material.

What is claimed is:
 1. A method of forming a fibrous composition,comprisingmixing in a liquid diluent carbon fibers and particles of a aperfluorinated high polymer resin to form a uniform slurry, filteringsaid slurry to form a filter cake having less than about 20 percentdiluent, compressing said filter cake at a pressure of from about 500 toabout 3000 psi, at a temperature of from about 100° F. to about 250° F.for a period of from about one to five minutes, passing said compressedfilter cake through the nip of heated rolls a plurality of times toreduce the thickness thereof and to orient substantially all of thefibers therein essentially parallel to the surface of said fibrouscomposition; heating the rolled composition to a sintering temperatureto bond the resin particles therein to form said fibrous composition. 2.The method of claim 1, wherein said rolled composition is placed betweenthe platens of a heated press at a temperature in the range of from 640°F. to about 740° F., at a pressure of from 50 psi to about 5000 psi fora period of from about one to about thirty minutes in order to pressuresinter said rolled composition.
 3. The method of claim 2, including theadditional step of moving said pressure sintered composition from saidheated press to a room temperature water bath.
 4. The method of claim 1,wherein said heated rolls are at a temperature of from about 100° F. toabout 250° F.
 5. The method of claim 1, wherein said compressed filtercake is passed through the nip of heated rolls a plurality of times,each pass reducing the thickness said filter cake from about 0.02 toabout 0.06 inch to obtain the desired thickness, andsubsequently heatingsaid rolls to a temperature of from about 280° F. to about 360° F. andthen passing said rolled filter cake a plurality of times, with saidrolled filter cake being folded double after each pass and passed againthrough the nip of said heated rolls.
 6. The method of claim 5, whereinthe folded double rolled filter cake is rotated 90 degrees to thedirection of rolling of the preceding rolling and passed again throughthe nip of said heated rolls.
 7. The method of claim 1, wherein saiddiluent is an isoparaffinic hydrocarbon.
 8. The method of claim 1,wherein said perfluorinated high polymer is polytetrafluoroethylene. 9.The method of claim 7, including the step of drying said rolled filtercake, prior to sintering, in order to evaporate residual diluentremaining therein.
 10. The method of claim 1, wherein said carbon fibersare graphite.
 11. A method of forming a fibrous composition,comprisingmixing in a liquid diluent a fibrous material and particles ofa perfluorinated high polymer resin to form a uniform slurry, filteringsaid slurry to form a filter cake having less than about 20 percentdiluent, compressing said filter cake at a pressure of from about 500 toabout 3000 psi, at a temperature of from about 100° F. to about 250° F.for a period of from about one to about five minutes, passing saidcompressed filter cake through the nip of heated rolls a plurality oftimes to reduce the thickness thereof and to orient substantially all ofthe fibers therein essentially parallel to the surface of said fibrouscomposition, heating the rolled composition to a sintering temperatureto bond the resin particles therein to form said fibrous composition.12. The method of claim 11, wherein said fibrous material ispolytetrafluoroethylene fibers, and mixed therewith particles of a highmolecular weight polyethylene resin.
 13. The method of claim 11, whereinsaid sintering step is conducted at a temperature in the range of fromabout 350° F. to about 400° F.
 14. The method of claim 11, wherein saidfibrous material includes graphite fibers and said sintering step isconducted at a temperature in the range of from about 500° F. to about540° F.
 15. The method of claim 11 wherein said perfluorinated highpolymer resin is polytetrafluoroethylene.